docs: def on_touch_up(self, touch):
          """Return to ``pos_start``, but first, save my current ``pos`` into
          ``pos_up``, so that the layout knows where to put the real
          :class:`board.Spot` or :class:`board.Pawn` instance.
  
          """
          if touch is not self._touch:
              return False
          self.pos_up = self.pos
          self.pos = self.pos_start
          self._touch = None
          return True